This invention relates to digital printers in general and in particular high resolution, color proofing laser printers with zoom lenses using multiple light beams in which the writing spot sizes and pitch can be varied.
Digital printing apparatus for color proofing have very high pixel densities, typically 2000 to 4000 spots per inch. Such high pixel densities require small pixels in the range of 1 to 30 microns in diameter and this in turn requires a print lens of high numerical aperture. (The term print lens here refers to the lens which focuses light to form individual pixels on a photosensitive medium.) It is desirable to have the capability to adjust the writing spot size and pitch of this type of system due to several widely used spot densities of 2400 and 2540 spots per inch. Also, in order to accurately simulate lower and higher resolution printers it is desirable to have color proofing printers with adjustable spot densities.
A lens designed for a fixed magnification producing a specific pixel density within a narrow range is disclosed in U.S. Pat. No. 5,258,777. The embodiments in this patent show the complexity of a print lens having image numerical apertures of 0.5 to 0.55 and which incorporate seven to nine elements.
Print lenses have other requirements such as telecentricity on the image side of the lens in order to minimize pixel pitch changes when the image focal distance changes as disclosed in U.S. Pat. No. 5,959,654. A lens telecentric in image space has image chief rays parallel to the optical axis as they exit the lens. A chief ray is the central ray of light within a focussed bundle and would be the only ray left if the aperture stop were to be closed to an infinitesimal opening. In certain cases, such as when a laser is used as the light source, the chief ray is determined by the light source. Another definition of chief ray is that the chief ray is located at the centroid or peak intensity of the image.
It is also sometimes necessary, depending on the light source, to have telecentricity on the object side of the lens. When object sources emit light with their chief rays in a parallel direction, the lens should be designed as a telecentric lens on the object side to avoid light loss for those sources not on the optical axis of the lens and to control off-axis aberrations. The fact that the entrance pupil is at infinity for a telecentric object must be recognized in the correction of off-axis aberrations of the print lens.
A lens which is telecentric in both object and image space must be afocal because collimated light from infinity exits the lens as collimated and does not come to a focus. A common application of afocal lenses is in telescopes where the object and image are effectively an infinite distance away. However, if the object is a finite distance from the lens an afocal optical system will form an image a finite distance from the lens. The use of afocal lenses for objects and images at close or finite distances is less commonly known.
U.S. Pat. No. 5,708,532 discloses a doubly telecentric series of lenses used for measurement at specific magnifications. At a magnification of xe2x88x921, this lens is symmetric, while at magnifications of xe2x88x920.5 and xe2x88x920.25, half the lens is replaced. This is one way to change magnifications, but it is too difficult for a print lens of high resolution clue to sensitivity of the lens position.
It is common practice to use zoom lenses to change magnification. In a zoom lens some lens element groups move and some stay in a fixed position. But in the design of an afocal zoom lens the difference between infinite and finite conjugates must be explicitly recognized because infinite conjugates require only two moving groups while a finite conjugate requires three moving groups. The difference comes with the finite conjugate requirement to hold the object to image distance as the lenses move. This additional constraint imposes the need for the additional degree of motion. Examples of infinite conjugate afocal zoom lenses are disclosed by Abraham in U.S. Pat. No. 5,783,798 and Cobb in U.S. Pat. No. 5,134,523. These are used to relay a laser beam with a diameter. Two other infinite conjugate afocal zoom lenses are disclosed by Minoura in U.S. Pat. No. 4,390,235 and Tokumitsu in U.S. Pat. No. 4,353,617.These last two are used in printer applications.
Nezu et al., U.S. Pat. No. 4,617,578 discloses a multiple beam zoom lens with an afocal section whose purpose is to adjust the pitch between lasers. This lens is not telecentric on the image side due to the nature of its design, and suffers pitch changes when the focal plane focus position changes, which limits the depth of focus. This approach therefore is less desirable due to the need for tighter manufacturing tolerances. A common error of setting manufacturing tolerances is to allocate some focal depth loss to each error.
Mizutani et al., U.S. Pat. No. 5,805,347, discloses a doubly telecentric lens formed with two positive and one afocal group. This design is overly restrictive in holding the internal group afocal during zoom. This invention discloses only the first order properties of the invention without disclosing the nature of the three groups in any detail leaving the reader unable to evaluate the image quality.
Wakimoto et al., U.S. Pat. No. 4,867,545, discloses telecentric systems with variable magnification. These inventions and their embodiments show either limited performance or are not true zooms. In the first case, the magnification range is small and the telecentricity and focus are not strictly held during zoom, however, the object and image distances are fixed. In the second case, the magnification range is large, but all the three groups move, so neither the object nor image position is fixed with respect to any component of the lens. The third problem with these lenses is that they all use three positive groups and therefore have too much field curvature.
In U.S. Pat. No. 5,414,561, Wakimoto et al. improves on their prior inventions by using a central negative group with the two outer positive groups. This helps to reduce the field curvature. Of the seven embodiments of this patent, only the third embodiment has constant object, image, and overall lens length during zoom. This lens works only at F/8 and is extremely long at over 0.5 meters.
It is therefore desirable to provide a multi-beam zoom lens for producing variable spot sizes in a laser printer. It is also desirable to provide a multi-beam zoom lens which has the capability of varying the pitch of writing spots.
According to one aspect of the present invention a multi-beam zoom lens for producing variable spot sizes on a photosensitive medium from a plurality of individually modulated light sources wherein each light source emits a light beam parallel to each of the other light sources and parallel to an optical axis and wherein a numerical aperture of each of said light beams is greater than 0.125, comprises an afocal zoom lens. The afocal zoom lens comprises a first moving group of lenses, a second group of moving lenses, and a third group of moving lenses. A constant barrel length of the afocal zoom lens is less than 160 mm and the zoom lens has a constant distance from the light sources to the photosensitive medium of less than 180 mm. The zoom lens has an afocal magnification of at least 45% across a zoom range.
According to one embodiment of the present invention multiple light beams in which the writing spot sizes and pitch can be varied are used. Each beam of light from a plurality of light sources is aimed so that its central axis, or chief ray, is parallel to the optical axis as it enters the lens. By design, the exiting beams have their chief rays also parallel to the optical axis of the lens, making the lens afocal throughout its zoom range. In addition, the lens works with fixed object and image distances while maintaining a fixed length throughout its zoom range. The numerical aperture of this lens is also very high varying from 0.29 to 0.41 (F/1.73 to F/1.21). This is much higher than prior art and makes it suitable for laser thermal printers in which high power is needed to expose the medium. The object to image distance of this system is only about 141 millimeters, much less than prior art.
This invention provides a means to vary the writing pixel size and pitch continuously by at least 50% using a doubly telecentric or afocal zoom lens. It also provides a means to keep the object to image distance and focus of the lens constant throughout the zoom range. The lens of this invention has a constant distance from the object to the first element along with a constant distance from the last lens to the image, and a constant lens length from the first to the last element. This is a significant advantage and allows the change of writing pixel size and pitch of the writing spots in the product without having to make any other adjustments, such as the focal distance, external to the lens.
A lens, according to this invention, also provides means to compensate for rotationally symmetric manufacturing errors in the lens by performing a one-time adjustment of one group""s internal position. Improvements in the present invention include: the ability to change pixel size and pitch by at least 50%, telecentricity on both the object and image side of the lens, a fixed lens barrel length, a fixed object and image distance, a high numerical aperture of F/1.7 to F/1.2, a compact length, and compensation means for rotationally symmetric manufacturing errors.